The invention relates to an improved process for the preparation of S-arylthioglycolic acids by reaction of aromatic diazonium salts with thioglycolic acid in the presence of copper salts, particularly those of divalent copper.
Processes for the preparation of S-arylthioglycolic acids by reaction of aromatic diazonium salts with thioglycolic acid have been known for decades. A process is described as early as German Patent Specification No. 194,040 in which aromatic diazonium salts are allowed to act on thioglycolic acid and the resulting reaction product, the aryldiazomercaptoacetic acid, is converted, directly or after previous isolation, into the S-arylthioglycolic acid by heating, nitrogen being split off.
In another process, described in German Patent Specification No. 201,231, the decomposition of the aryldiazomercaptoacetic acid to the S-arylthioglycolic acids is carried out at temperatures of 20.degree. to 40.degree. C. by addition of copper powder.
Furthermore, the process of German Patent Specification No. 201,232 is known, by which the decomposition to the S-arylthioglycolic acids is carried out by heating in alkaline solution.
In addition, a process for the preparation of 2,3-dichlorophenylthioglycolic acid is described in Swiss Patent Specification No. 451,921, Example 1, in which the reaction of the aromatic diazonium salt with the thioglycolic acid is carried out in alkaline solution at 0.degree. to 5.degree. C.
The disadvantage of all these processes is that yields of only 20 to 50% can be obtained, and the S-arylthioglycolic acids produced are very impure. In addition, an industrial operation in which an aryldiazomercaptoacetic acid, which tends very readily to decompose very violently, arises as an intermediate is very problematical.